Loft condensation

[Andrew Gabriel]

Interesting problem with loft condensation...

100 year old house, no water/plumbing in the loft. Loft is lined and retiled, probably 40+ years ago. Loft has always been bone dry, at least through to last summer. Loft is well sealed from the house, but there's pretty much no moisture source in the house anyway (far less than there would normally be due to lifestyle of the occupant).

Last summer, various maintenance activities were carried out around the loft and roof.

Some lead flashing was replaced, and that still seems to be fine - no water penetration. (Prior to that, a small amount of rain water was running down the chimney breast in the loft, but it wasn't actually causing any problems, and loft was otherwise bone dry.) Chimney breast is now bone dry, so that's fixed. Indeed, there's no sign of rain penetration anywhere else either.

Another 4 inches of loft insulation were added, on top of the 2-3" that have been there > 25 years.

I replaced the facia and gutters along one side. There was no ventilation there before (house has no soffits), and I spaced the new facia a little bit off the wall plate timber to allow a little ventilation up between them.

Whilst there was access to the chimney, I took opportunity to add more ventilation in the form of air bricks in the gable end (one large, one smaller), as the loft wouldn't meet modern ventilation requirements, although as I said before, it had never got damp before.

Just checked it after the winter, and it's obvious that the rafters have all been very wet over winter, which has never happened before. It's spread across the whole roof, so it's condensation, not a leak. Being an old house with untreated timbers, I can't let this continue.

So I'm trying to work out the cause, and it would seem to match something I have hypothesised before, and this is what I think has happened.

Before I did the work, there was a little loft ventilation, and little loft insulation. Heat would leak from the house, warm the loft, and consequently reduce the relative humidity of the (outside) air in the loft so there was no chance of any surfaces lower than the dew point, as the loft will be warmer than the outside air.

Now, the increased ventilation means more outside air is passing through the loft. The increased insulation means the loft isn't being warmed. On cold clear nights, exposed surfaces such as a roof radiate heat and in the absence of any heat source, cool below the outside air temperature. This will now drop below the dew point of the outside air, and form dew/condensation on the inside of the roof, in the same situation you get dew outside.

Does this sound plausible?

Any bright ideas on fixing it, apart from rolling up the loft insulation and blocking off the new air vents? It seems to me such a roof is dependent on heat leaking from the house.

[andrew]

It does, it also sounds similar to the problem I have with tin roof.

What we talked about then was moving a thin layer of insulation tight to the roof surface to isolate the radiating roof from the air space. I still haven't solved our problem yet but I suspect the RH was lower in our extended cold spell compared with last year.

AJH

[Roger Chapman]

Yes. (But see below).

Are you sure that there is no leakage of air from the main part of the house into the loft. Your increased ventilation may have the affect of providing a pressure difference between house and loft and an influx of warm humid air into a cold space will certainly cause condensation.

There will be some slight transfer of heat to the loft however deep you make the insulation so I have my doubts about the roof managing to radiate down to below ambient. Grass frosts typically occur below 3C but the grass is almost isolated from the ground and even with the limited heat coming up from the earth air temperature above bare ground is much closer to zero before that attracts a frost coating.

With a similar problem in the past I have used EasyLog data loggers to establish the extent of the problem but that was transient rather than continuous through the winter. (EL-USB-lite IIRC).

[Andrew Gabriel]

There are no holes punched through the ceiling. The loft hatch and ceiling isn't going to be completely air-tight of course, but it's no worse than it was for previous 25 years. Nothing has changed in the house either regards producing moisture.

The house has temperature logging, but not in the loft. So, I just added a sensor in the loft and I'll compare it with the outside sensor. Having come out of the loft and closed the hatch, the loft temperature is rapidly dropping towards outside temperature, but too early to know where it will settle yet.

It looks like we might get a couple of nights at 0C this week, but not with clear skys, but I'll see what readings I get.

[Andrew Gabriel]

I did, and that's when the problem appeared.

Yes, about 10 years ago. The chimney is however only vented to the outside at the base, not to the inside. Now that the chimney is properly ventilated at the top, I can easy block off the vent in the loft; it was a temporary measure until I could put a proper vent at the top of the stack. I struggle to see how it would cause the observed effect over the last winter though.

[Roger Chapman]

snip

Temperature is only half the equation. Do you have a dehumidifer to put up there for a bit to see how much water you can extract at say 80% humidity?

[andrew]

The hypothesis is that the roof is a good radiator and insulated from the building and ground. Air passes through the roof space. As the air cools faster than the surroundings barring those that are good radiators and not able to conduct heat from the ground ( cars and windscreens being examples) it approaches saturation. This saturated air passes into the roof space and out the other vents but on the way it contacts the cold roof underside and reaches its dewpoint. It's a small matter if there is only one air change in the night, maybe 10 grams per m3 which would evaporate next day, but if the space is well ventilated...

AJH

[The Natural Philosopher]

Yes but...

Do you have loft mounted tanks that are un-pressurised?

I have a modern roof that is precisely similar, but there is no damp at all.

BUT there is almost a hermetic seal from downstairs.. and a few heats sources in the shape of loft mounted lighting transformers, that overheated in the insulation, that now lie on top of the fully boarded structure.

open header tanks that are kept well above freeing can be a huge source of moisture.

[David Robinson]

I've seen Andrew G propose his theory before, and it's a shame in a way that it may have proven true in practice.

It doesn't seem like letting heat into the loft is seen as a good thing. It doesn't seem like blocking up all the ventilation is seen as a good thing. So how else can the loft surfaces be prevented from acting like a car and its windscreen (and attracting condensation and frost)?

Or, to put it another way - how come all modern houses (lots of insulation, lots of ventilation) don't face this problem?

Cheers, David.

[The Natural Philosopher]

The frost or dew on a car disappears the moment the sun hits it: Its really not a long term issue.

Internal misting happens more because the car is left with a sticky steamy interior after last nights drive.. if a car is left for LONG periods it doesn't suffer from internal condensation.

In short, any ventilated structure that is weather proof - a barn or a shed- does not suffer rot until its inhabited and steamy breath raises the RH.

[george [dicegeorge]]

It was a very very cold winter with a huge temperature difference between the house and the loft space, I guess warm air is trickling up there from downstairs somehow - the chimney? gaps at the edges? permeating through the rockwool? If there's a pressure difference it will find a way. I dont think a few weeks condensation in December will cause rot, has it dried out now? [g]

[Andrew Gabriel]

I do have one, but they don't work at the current loft temp (6C) as the evaporator ices up. I could perhaps put it upstairs with the outlet ducted up into the loft as a measure to dry it out quickly.

I have some readings now. Last night and this morning, loft stays

2C higher than outside temperature. It's been continuously cloudly over that period, so no radiative cooling or heating to speak of, and no wind to speak of.

I varied upstairs temperature from 16.5C to 20.5C over night and it made no difference to loft temperature, but this would probably need running over a longer period to get a noticable effect.

This morning, I put 500W of heating in the loft to see what impact that has on the temperature. That raises the loft by a further

4C, to 6C above outside. Very conveniently, the outside temperature hasn't changed over this period, so the temperature rise should be proportional to the heat loss, so that implies that I'm normally losing about 250W through the new loft insulation. I don't have equivalent measurements before doing the extra insulation, but I would imagine it could well be many times that heat loss.

Another interesting observation. In the loft is a large metal tray,

4' x 3' (originally the side from a 19" rack). It was originally supported above the new insulation, but the insulation has expanded a bit since laying, and was now touching the bottom. There is a little condensation on the bottom of this, which implies some moisture is working it's way through the ceiling, which is lath and plaster at that point, but not cracked. I still find it hard to believe all the moisture in the loft is coming from the house though.

[george [dicegeorge]]

warm moist air can get between my floorbaords then up the internal lathe and plaster walls and then into the loft. [g]

[Andrew Gabriel]

No, still wet, and smells wet too.

[Skipweasel]

Or, in the case of many cars, 'cos the driver doesn't understand how to set the fresh/recirculate lever.

Vauxhall seem to be to be particularly prone to this - the number of Corsas and Astras you see driving along that look like someone's having a sauna in 'em.

[The Natural Philosopher]

Are you saying that you have no vapour barrier underneath the insulation? and that it's jut lath, plaster and rockwool there, alone?

[Tim Streater]

[snip]

Arrrhhh, are you saying there should be something? So you don't just put the fibre-glass or whatever between the joists and run away?

[Tim Streater]

What are shower cubicles?

There are extractor vents in the bathrooms. The kitchen extractor hood unfortunately doesn't vent to the outside like it should.

[Roger Chapman]

That sounds very much as if the relative humidity rarely drops much below 100%.

[Terry Fields]

My 2003 house has breathable sarkin. I expect the roof is ventilated but it isn't obvious how this is done. There's no sign of condensation anywhere in the loft space, which has ~100mm fibreglass insulation.

I read an article many years ago in the New Scientist, that looked at the deterioration of unused/abandonded stately homes. The article concluded that the most significant destructive mode was humidity cycling. Apparently, IIRC, cycling to less than about 45 percent caused wood to shrink and paint to peel off, and above about 60 percent caused fungal spores to become active.

I'm certain the article concluded that the best way to keep the humidity levels under some sort of control was to keep the interior 5 degC above the outside temperature. It just might be that thicknesses of layers of loft insulation less than that currently recommended might do just that, and be the price one pays for a damp-free loft space.

I've never seen a calculation that estimated the water-vapour diffusion rate from upstairs rooms at say 20 degC and 50 percent RH into a loft at 5 degC and say 90 percent RH, but the OP's posting suggests that it might be greater than desirable.

TF